The present invention relates to a power transmission device for a vehicle, and, more particularly, relates to a power transmission device for a vehicle with two axially opposed driven wheels, in which a differential is provided between these driven wheels and is axially located closer to one of these wheels and further from the other of them--i.e. in an asymmetrical location relative to the wheels.
In the case of a vehicle provided with a transversely mounted engine, especially in the case of a vehicle of the front engine front wheel drive type or the so called FF type, it is very common for the differential device which transmits power from the gearbox of the vehicle to the driven wheels thereof, and which provides differential action between said driven wheels, to be displaced to one side or to the other side of the longitudinal axis of the vehicle, i.e. for this differential device to be closer to one of the driven wheels and farther from the other of the driven wheels. This is because of the limited space available in the engine compartment of such a vehicle, due to the transverse mounting of the engine therein, and to the relative alignment of the various units in the power train of the vehicle, and is a well known matter in the automotive art.
The question then arises as to how the rotary power is to be transmitted from such an asymmetrically positioned differential to these driven wheels of the vehicle. If the differential is provided with left and right power output shafts which are of the same length, and if the end of each of these power output shafts is connected to its respective driven wheel by a drive shaft and by universal joints, then, since the left and the right drive shafts will necessarily be of different lengths, these drive shafts will incline downwards at different angles, and will be connected to the driven wheels through different angles. When the drive shaft is inclined downward at an angle, it exerts during acceleration of its rotation such a moment on the driven wheel as will steer the driven wheel inward according to the amount of the angle of downward inclination. This means that during acceleration of the driven wheels, and also, when the driven wheels are the front wheels of the vehicle and are also used for steering the vehicle, the difference in the angles of downward inclination of the left and the right drive shafts will cause asymmetrical acceleration and steering characteristics for the vehicle, so that a tendency will arise for the vehicle, during acceleration, to steer towards the side on which the drive shaft is longer. This deteriorates straight ahead drivability of the vehicle, and accordingly is quite unacceptable.
In view of the above described problem, a constructional solution has been proposed for a power transmission with such an asymmetrically located differential, in which the left and right drive shafts are of equal lengths, and incline downwards at the same angles, and on the side of the vehicle on which the distance between the differential and the driven wheel is the shorter the inner end of the drive shaft is directly connected to the outer end of the power output shaft of the differential by a universal joint, while on the side of the vehicle on which the distance between the differential and the driven wheel is the greater the inner end of the drive shaft is connected by a universal joint to the outer end of an intermediate shaft, the inner end of which is connected, possibly by another universal joint, to the outer end of the power output shaft of the differential on that side. In this case, the outer end remote from the differential of this intermediate shaft is often required to be supported, via at least one bearing, by some fixe member of the vehicle such as the engine thereof.
In such a construction, the angles down through which the left and the right drive shafts incline are the same, and accordingly the left and right driven wheels are provided with equal steering torques, accordingly eliminating asymmetrical steering and driving characteristics of the vehicle; on the other hand, it is important to make the support rigidity provided by the aforesaid support bearing which supports the outer end remote from the differential of the intermediate shaft rather high. Now in this case it would be ideal if the central axis of this support bearing could be aligned, during the fixing of the support bearing to the member by which it is supported such as the engine of the vehicle, so as to be coincident with the axis of the output shaft of the differential on that side; but in practice, due to the inevitable effects of manufacturing tolerances and errors, this is not possible, and an offset between these two axes, as well as non parallelism thereof, always occurs during manufacture and assembly.
Because, therefore, it is very difficult to align the central axis of the support bearing with the central axis of the output shaft of the differential on that side, the problem arises that high stresses are set up during assembly and are maintained during vehicle operation, both on the components of the support bearing and on the components of the differential. In particular, an offset load on the support bearing means that the operating life thereof will be significantly reduced. Especially if this support bearing is a radial ball bearing of a conventional sort, if it is fitted with significant displacement present between its central axis and the central axis of the output shaft of the differential on that side, the inclination thereby caused of the axis of the intermediate shaft relative to the central axis of the outer race of the support bearing means that the inner race of said support bearing will be forced to be inclined with respect to the outer race, and this will make the life of the support bearing rather short. Thus, the reliability of the transmission as a whole will be decreased.